What the RC hobby people do for this is to get a 6.0 - 7.4 V UBEC and power the servo off that, the UBEC off the battery. An UBEC is really just a switching voltage converter.You can find 7.2V / 3A UBECs for < $10 from the various cheap hobby sources, and probably would pay up to $20 from the highest-end brands/stores.